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Fender "Evil Twin" Red Knob sounding... wonky in low-power mode


dthines84

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One of my current projects is restoring this 1988 Evil Twin that I picked up super cheap. The original chassis was beyond redemption, with blown caps and burned out PCB's, so I replaced the whole thing, re-tubed it, and fixed up all of the obvious issues. On High power mode (with two 6l6's removed to operate in 60w RMS) it sounds beautiful, crisp and bell-like. However, when I switch it to low power mode, (running 15w RMS) it sounds horrible, like theres some sort of microtonal distortion/feedback or something. I can't seem to track down the culprit for this issue, any ideas of where to start trouble-shooting?

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My GUESS is crossover distortion from mis-biased tubes. Apparently the low power mode is similar to my Music Man amp, the switch just selects a lower B+ voltage from a tap on the power transformer. Just stick those other tubes back in, it's only 25 watts in 'lo power' mode.

 

 

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You're likely dropping the power too low by only using two tubes. The low power switch when running 4 tubes drops the power from 100W to 25W (6.25W per tube) which means 4 tubes are already running about as low as you cant get them without them dipping into cutoff.

 

You have to understand how tubes work in order to understand why its distorting, Most guitar players understand pushing tubes too hard will cause them to compress and distort. Tubes do something similar (but sound allot nastier) when they are starved for bias.

 

Normally you run a tube within its center of amplification range for best fidelity so the peaks don't hit the ceiling and clip or hit the floor and shut off.

In an AB amp they keep a steady minimum bias on the tubes so they don't shut off and act like a switch (and motorboat) This bias uses up the ability to run the tubes below a minimum wattage level.

 

You can see here the arrow pointing at the blue line saying, slightly over Half Cycles. That's the minimum level for a single tube in an A/B circuit. The second tube does the same thing except its a polar opposite phased in reverse. This pic shows the wave going north, then nothing between it and the next north going cycle. The second power tube is taking care of the south swing and the pair create a full sine wave.

 

class-AB.gif

 

 

 

 

When you pulled the tubes you already cut the peaks in half. The tubes will likely work fine on the 25W setting if all 4 power tubes are being used. Each tube will be providing about 6.25W that way.

 

When you run a pair you have 50W. When the 1/4 power switch is used the bias isn't right for only two tubes. You're running in the cutoff zone.

 

You may be able to change the voltage to the switch so it only cuts the bias voltage in half for two tubes instead or 1/4 for 4 tubes, but My best suggestion is, since 2 tubes is 50W - If you want lower wattages, put all 4 tubes back in and run the amp in the 25W mode.

 

That's half as much as running it with two tubes. If you play out, and need to kick it up, the dB level increase from 25W to 100 isn't nearly as great as 0 to 25W. otherwise you'll need to convert the amp to a 50W amp, get a 50W transformer. That 100W transformer will stretch to 50 or even 25W but its not right for getting much lower then that. You got to redesign the thing or simply sell it and buy something else more suitable.

 

I can say I'm not a huge fan of bias drop switched. They kill the tone with 6L6 tubes. They do better with EL34 tubes. My Sound City and Music man amps don't suffer too much when run at 1/2 power but of the amp. I have a Sunn City and a Music Man with that feature and out of the two the Sound City sounds the best, simply because the tubes are run at max wattage on the high power (60W) and they drop to maybe 45W on the low power and have plenty of fidelity in that range. You get down lower and its mud city.

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That's what I'd do. See how the 25W mode works. That 100W transformer is just too big. It will allow you to run the amp with 2 tubes but the bias cut is designed for 4 tubes not 2.

 

If you want a cranking low wattage amp you should think about buying those yellow jackets which allow you to run low wattage tubes.

https://www.tubesandmore.com/products/yellow_jackets Or have the amp converted to 6V6 tubes so its around 60W and drops to 15W with the bias switch.

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Damn' date=' Mr. Grumpy. I feel like an idiot now, I totally forgot to bias and balance after re-tubing. I'll go put on my dunce cap and fix it. Thanks[/quote']

 

You should also check the integrety of the 1 Ohm resistors between the cathodes of the power tubes and ground.

 

They provide a convienient way of measuring the current flowing through the tubes when adjusting the bias but they are also a weak spot on those particular amplifiers. A 40 mV drop across a 1 Ohm resistor indicates 40 mA of current flow.

 

If the resistor changes its value, which happens sometimes, it will upset the balance of the push pull circuit and cause some ugly distortion - which is particularly noticable at lower volumes.

 

If you are careful, you can check the resistors without removing the chassis by using an Ohmmeter connected to the to the test points used for setting the bias. By careful, I mean disconnect the amplifier from power and remove the power tubes. You may need to reverse the leads on your Ohmmeter to get an accurate reading because the 1 Ohm resistors have a diode in parallel with them

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